1. Field of the Invention
This invention relates to an improved method for the formation of a substantially interwoven matrix containing refractory hard metal. More particularly, this invention relates to an improved method of forming a substantially interwoven matrix containing a refractory hard metal with little or no external heat input. The resultant product is subject to less distortion during formation, thus rendering it more suitable to applications, such as a cathode for a reduction cell for the electrolytic production of metal, such as aluminum.
2. Description of the Prior Art
Refractory hard metals, such as TiB.sub.2 or ZrB.sub.2 are useful in applications where a material is needed possessing both mechanical strength and chemical inertness, particularly at elevated temperatures. Such uses include use as an electrode, e.g., a cathode, in an electrolytic reduction cell for the production of metal, such as aluminum.
When aluminum is produced in a Hall cell, for example, a certain amount of carbon is consumed from the electrodes per pound of aluminum produced. In Hall cells, often the cathode is fabricated from carbon or graphite, and in a typical Hall cell, the cathode is covered with a layer of aluminum. The layer of aluminum is maintained on the cathode for several reasons. First, aluminum reacts with the carbon cathode to a certain extent to form a thin layer of aluminum carbide on the cathode. The aluminum carbide layer, upon exposure to the bath, e.g., cryolite, dissolves. The exposure can occur as a result of magnetic currents in the cell moving the aluminum layer leaving the aluminum carbide exposed to the bath. Thus, to minimize replacement of the carbon cathode, sufficient aluminum is permitted to remain on the cathode to prevent such wear. In addition, because of the molten metal movement as a result of the magnetic currents, the anode-cathode distance in the cell cannot be minimized since such can result in short-circuiting of the cell. Thus, it can be seen that normally such a cell is operated at less than its optimum efficiency because a larger anode-cathode distance has to be used.
In the prior art, many attempts have been made to overcome this problem of having an excessive anode-cathode distance. For example, it has been recognized that when TiB.sub.2 is used as a cathode surface, aluminum may be drained from the cathode surface without attack by the bath on the cathode. This, in turn, permits much closer anode-cathode distance without fear of short circuiting. However, the use of TiB.sub.2 having the high level of purity required in such an application has provided to be very expensive. If inferior grades are used, this results in cracking of the TiB.sub.2 and the attendant problems therewith. Of course, it will be recognized that one of the advantages of the use of refractory hard metals, such as titanium diboride, as a cathode surface resides in the fact that the TiB.sub.2 surface is readily wettable by liquid metal, e.g., liquid aluminum. This result permits the cathode surface to be drained of the liquid metal and eliminates the electromagnetic problems encountered with a deeper or thicker aluminum pool. This, in turn, permits the use of a smaller anode-cathode distance with a distinct advantage in power efficiency.
However, as noted earlier, special controls often are used to preclude failures in the use of refractory hard metals, e.g., TiB.sub.2, in these environments, such as, for example, set forth in U.S. Pat. Nos. 4,308,113 and 4,308,114. Also, U.S. Pat. No. 4,376,029 illustrates another attempt at using TiB.sub.2 as a cathode by using a TiB.sub.2 -carbon composite. One benefit disclosed for this composite is the fact that a cheaper grade of TiB.sub.2 may be used thereby lowering the overall cost of the cathode.
The interest in the use of TiB.sub.2 as an electrode or electrode surface for an electrolytic cell generated great interest in ways to economically produce such product in a form that could be used as a cathode. For example, U.S. Pat. No. 4,353,885 discloses forming TiB.sub.2 by vapor phase reaction. Additionally, U.S. Pat. No. 3,016,288 discloses an aluminothermic process of making boron compounds and alloys.
Merzhanov et al U.S. Pat. No. 3,726,643 describes the use of ignition means to react group IV, V, and VI metals with N, C, Si, O.sub.2, P, S, F, or Cl.sub.2 which comprises using an electric coil to ignite an ignition composition or agent. The ignition composition comprises Ca Si.sub.2, Fe.sub.2 O.sub.3, Mg, Al.sub.2 O.sub.3, SiO.sub.2, Ti, and B and is prepared from five mixtures of some of the ingredients which are then pressed together and ignited. The patentees describe the production of titanium diboride by mixing the ignition composition with metallic titanium and boron and then igniting the mixture.
J. Birch Holt in "Exothermic Process Yields Refractory Nitride Materials", published in Industrial Research and Development, April 1983, at pp. 88-91, describes an exothermic process for producing refractory metal nitrides such as TiN from titanium metal powder mixed with NaN.sub.3 and then ignited with an electric coil. In the article it is stated that this process, which takes advantage of the high heat formation of many compounds, and has been designated the Self-Propagating High-Temperature Synthesis (SHS), has been used to form borides, carbides, hydrides, silicides, nitrides, and other compounds of many different metals.
Yet, in spite of all this work, the use of TiB.sub.2 cathodes in electrolytic cells finds extremely limited use. One factor which limits its use is the high cost. Thus, in order to obtain the benefits of such a material, it can be seen that there is a great need for a process which would provide TiB.sub.2 or a material having properties similar to TiB.sub.2 at an economically feasible cost.
In my parent U.S. patent application Ser. No. 604,913, there is disclosed a novel composition in which a refractory hard metal, such as TiB.sub.2, is formed in an interwoven matrix with a metal compound by a displacement reaction. This invention relates to improvements in the method of making such a substantially interwoven matrix composition.